May 8, 2009
North and south were out of synch
The Mediaeval Warming Period and Little Ice Age features of past climate in the Northern Hemisphere are backed up by extensive evidence but there is much less data on palaeoclimate available for the Southern Hemisphere. As a result it hasn't been clear whether these phenomena extended across the globe.
Now a team from the US, France, New Zealand, Norway and Switzerland has come up with a technique for precisely measuring the date at which glaciers in New Zealand's Southern Alps began to retreat. Their study indicates that climate in the southern hemisphere was frequently out of step with climate further north over the last 7,000 years or so, during the Holocene period.
"The robust finding of our work is that the glaciers in New Zealand were out of phase with those in the northern hemisphere – Swiss Alps and Scandinavia," Joerg Schaefer of the Lamont-Doherty Earth Observatory told environmentalresearchweb. "This holds true in particular for the prominent northern climate feature labelled the Medieval Warm Period-Little Ice Age interval, the former being a warm period between about 900–1100 AD and the latter being the dominant cold in the north during the Holocene. This result argues for the importance of regional climate drivers, and we suggest changes in atmospheric circulation patterns to be important."
Schaefer and colleagues used a cosmogenic dating technique to analyse glacier fluctuations. By measuring the concentration of beryllium-10 in the moraines deposited by the glaciers they were able to date the glacier's retreat. Beryllium-10 forms when cosmic rays hit rock and concentrations gradually build up over time.
"New Zealand has one of the best resolved and preserved moraine records worldwide, and our team refined the cosmogenic dating method to a degree that we now can date even the young Holocene moraines," said Schaefer. "Holocene climate signals are small in amplitude, [and] very hard to detect in any climate archive. Glaciers are extremely sensitive to such small changes, and even the small climate swings of the Holocene forced the glaciers to fluctuate signficantly, which is documented in the moraine record – sediment ridges the glacier piles up while the glacier is steady in one position, i.e. in equilibrium with the prevailing climate."
The researchers found that New Zealand's Mueller, Hooker and Tasman valley glaciers have fluctuated frequently over the last 7000 years, with glacial advances becoming slightly smaller over time. Glaciers around Mount Cook reached their largest extent around 6,500 years ago when the Swiss Alps and Scandinavia were relatively warm. Northern glaciers, on the other hand, hit their peak roughly 6000 years later, during the Little Ice Age of 1300–1860 AD. New Zealand glaciers showed additional peaks during the northern hemisphere's Mediaeval Warming Period, just before the Little Ice Age.
"Our results also manifest how frequently the southern glaciers fluctuated during the rather stable Holocene," said Schaefer. "This illustration of the outstanding sensitivity of glaciers to small climate changes is a worrying finding giving the ongoing climate warming."
The researchers reckon atmospheric circulation changes in the southwest Pacific were a factor in forcing the glacier fluctuations.
Now the team would like to use its dating technique to create a global map of glacier fluctuations that would "provide information about climate changes on the continents we do not have yet, and would, in turn, provide a rigorous calibration data set for any climate models".
"This is what we have in mind for the near future in an effort that certainly includes the wider community due to its very ambitious scope," said Schaefer. "But I am very optimistic that we can get this done."
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of environmentalresearchweb.